Diagnostic Studies. Plain radiographs show a relatively radiolucent, bone-forming (osteoblastic) lesion that is osteolytic and often has an aneurysmal, or blown-out, appearance. It is surrounded by a thin margin of reactive bone that frequently extends into adjacent soft tissue. The intense bony reaction around osteoid osteoma does not occur with osteoblastoma.

The radiographic differential diagnosis includes osteoid osteoma (see Plate 6-2), aneurysmal bone cyst (see Plate 6-11), eosinophilic granuloma (see Plate 6-10), giant cell tumor of bone (see Plate 6-13), and osteosarcoma (see Plates 6-15 and 6-16).

Bone scans demonstrate an intense radioisotope uptake, but MRI helps to determine the surgical approach.

The histologic appearance of osteoblastoma is quite similar to that of osteoid osteoma but the former has a more prominent vascular component. In addition, osteoblastoma has more stromal tissue and giant cells and broader osteoid seams than osteoid osteoma. These characteristics also distinguish osteoblastoma from osteosarcoma. However, the distinction of osteoblastoma from osteosarcoma is not always clear. For example, osteoblastomas often contain scattered mitotic figures and a proliferation of immature osteoblasts.

Treatment/Prognosis. Osteoblastomas can recur after intracapsular procedures, which are usually done to preserve joint function particularly in the spine.

Some cases of malignant transformation of osteoblastoma into osteosarcoma have been reported, and it is not clear whether this is due to the difficulty in initial diagnosis or genuine transformation. Regardless, patients with osteoblastoma must be observed not only for recurrence but also for malignant transformation.